1. Field of the Invention
The present invention relates to a step-up/step-down switching power supply that is used in electronic equipment, etc., which employs batteries as the power source, and that provides output voltages which are higher or lower than the battery voltage.
2. Description of the Related Art
Switching power supplies have come into wide use in recent years in electronic equipment using batteries as the power source, for the purposes of operating such equipment with a smaller number of batteries and of extending battery life. We now describe one example of a conventional switching power supply, as noted above, making reference to the drawings. In FIG. 8, which is a circuit diagram of a conventional switching power supply, a transistor Q51 is turned on and off by pulses input from a comparator 7 through a resistor R53. First of all, when transistor Q51 is turned on, current flows to transistor Q51 from a battery 1 through the primary coil L51a of a flyback coil L51. At this time, the charge stored in a smoothing capacitor C3 flows through a diode D51 and a resistor R51 to charge a capacitor C51. Next, when Q51 turns off, the voltage on the collector of transistor Q51 becomes a higher voltage with the rise on the other terminal of a secondary coil 51b due to the flyback of the flyback coil L51 and due to the addition of the C51 voltage, and flows through a resistor R52 into the base of a transistor Q52. Transistor Q52, therefore, turns on, and, while transistor Q51 is on, the energy stored in the primary coil L51a of L51 passes through transistor Q52 and is stored on capacitor C3. Diode D51 here is for the purpose of preventing the charge on capacitor C51 from flowing back to capacitor C3 so that it will effectively flow into the base of transistor Q52. Capacitor C2 is for the purpose of absorbing the ripple current that flows into the flyback coil L51. A capacitor C52 is for the purpose of preventing oscillation in transistor Q52.
Next, the voltage on the capacitor C3--that is, the voltage applied to a load circuit 13 that operates the electronic equipment using the batteries--is divided by a series circuit made up of a resistor R1, a variable resistor VR1, and a resistor R2, and is amplified by an operational amplifier 2 to which the difference in voltage relative to a reference voltage source 3 has been fed back. The purposes of the capacitor C1 are to increase the low-band gain and reduce steady-state deviation, and to have the high-pass gain limited by a resistor R3 so that it does not oscillate. Next, the output from the operational amplifier 2 is compared by a comparator 7 with the triangular wave that is the output of a triangular-wave generator circuit 51, pulse-width-modulated (hereinafter abbreviated PWM), and made to drive transistor Q51 through resistor R53.
Here, the voltage is controlled in the following way. When the voltage of the battery 1 is lower than the load voltage across both terminals of capacitor C3, transistor Q51 is PWM-driven with a duty corresponding to that voltage difference, and operates as a step-up switching power supply, whereas, when the voltage of the battery 1 is higher than the load voltage across both terminals of the capacitor C3, the PWM duty becomes smaller, the charge stored in capacitor C51 decreases, the base current at transistor Q52 drops, and the potential difference between the collector and emitter in transistor Q52 opens up.
Besides this, a switching power supply is disclosed in the specification of laid-open patent application No. H2-51357 (1990).